Electrical submersible well pumps (“ESP”) are commonly used to produce well fluid from hydrocarbon producing wells. A conventional ESP has a pump operatively coupled to an electrical motor for driving the pump. A pressure equalizer or seal section is normally located between the motor and the pump. One common type of pump is a centrifugal pump.
A centrifugal well pump has a large number of stages, each stage having a rotating impeller and a non rotating diffuser. The impeller has a tubular hub through which the pump shaft extends. Impeller vanes extend outward from a central intake area. Top and bottom shrouds are mounted to upper and lower edges of the vanes to define impeller passages. The impeller has a downward extending cylindrical skirt that engages in a sliding fit with a skirt guide on an upper side of the diffuser immediately below. As the shaft rotates the impellers, the well fluid discharged upward creates a down thrust on each impeller, the down thrust being absorbed by a thrust washer between the bottom shroud and the diffuser immediately below.
Some wells produce considerable quantities of abrasive particles such as sand. The abrasive particles cause wear on various surfaces of the impellers and diffusers. One place of wear occurs on the skirt, creating an annular clearance between the skirt seal area and the mating surface on the next lower diffuser. The increasing annular clearance increases leakage and reduces the differential pressure across the skirt seal area. With a lower pressure differential, the pressure acting upward on the bottom shroud reduces, increasing the down thrust of the impeller. The increased down thrust can lead to production loss.